A backlight module is an important component of Liquid Crystal Display (LCD) devices. As liquid crystals do not emit light by themselves, a backlight module is mainly configured for providing an even and high-brightness illuminant for a liquid crystal panel. The fundamental principle of the backlight module is to convert a dot or linear illuminant to a surface illuminant with high-brightness and homogenous brilliance through an effective optical mechanism, such that the liquid crystal panel may display images normally. Backlight modules may also be used in digital photoframes, E-papers, mobile phones and other display devices requiring backlight, in addition to liquid crystal televisions and LCDs.
A schematic cross section view of a backlight module used in a LCD is illustrated in FIG. 1. The backlight module comprises: a backplate 1, a reflector 6 disposed on the backplate 1, a light guide plate (LGP) 3 disposed on the reflector 6, as well as optical films such as a prism sheet and a reflective polarizing film (not shown in FIG. 1) disposed on the LGP 3. A light source 11 is disposed on a side of the LGP 3. The light source 11 comprises a plurality of Light Emitting Diodes (LEDs), which are fixed to a flexible printed circuit board 8 (FPCB). Light emitted by the light source 11 enters the LGP 3 at a side of the LGP 3 immediately adjacent the light source. Light entering the LGP 3 exits from an upper side of the LGP 3 and enters the optical films to be converted into backlight with high-brightness and homogenous brilliance. To obtain higher light extraction efficiency for the LEDs, the LGP 3 has to contact the LEDs very closely. As illustrated in FIG. 2, according to conventional arts, two transparent double-sided adhesive tapes are mainly used to fix the LGP 3 to the backplate 1, where one double-sided adhesive tape (i.e., a double-sided adhesive tape 5) is attached to an end of the LGP 3 close to the LEDs, while another double-sided adhesive tape (a double-sided adhesive tape 4) is attached to another end of the LGP 3 away from the LGP3, with a length approximately equal to a width of the LGP 3.
It is found that the light source is often at higher temperature due to poor heat dissipation, long operation hours and so on. If the above light source works at a high temperature for a period of time (say for two hours at 50° C.), brightness of the liquid crystal panel will be decreased after the light source returns to its normal operation temperature. Such phenomenon is due to the following fact: when the light source functions at a high temperature, the LEDs are the main heat source. As a result, the end of the LGP 3 close to the LEDs is expanded quite severely, causing the LGP 3 to extend further away from the LEDs. The expanded and extended LGP 3 will be fixed once again at a new position by the two double-sided adhesive tapes. On the other hand, the double-sided adhesive tape 5 close to the LEDs ages easily by the high temperature, leading to a smaller fixing force than the double-sided adhesive tape 4 away from the LEDs. As a result, when the light source cools down, the LGP 3 will contract, with the double-sided adhesive tape 4 away from the LEDs as a fixed point, causing a gap between the cooled-down light source 11 and the LGP 3. The presence of the gap will affect the light extraction efficiency of the LEDs, which will further decrease the brightness of the display.